Polyamide resins are extensively used as engineering plastics because of their advantages in stiffness, abrasion resistance, chemical resistance, heat resistance, and electric properties. Polyamide resins, however, have disadvantages in low impact resistance, and deterioration of physical properties on moisture absorption, which are required to be improved.
On the other hand, ethylene copolymers such as an ethylene-acrylic ester-maleic anhydride copolymer have advantages of flexibility suitable for a rubber elastomer, high impact strength, no hygroscopicity, satisfactory forming characteristics, and sufficient adhesiveness to metals and polyamide resins. Ethylene copolymers, however, are inferior in abrasion resistance, chemical resistance, and heat resistance.
As described above, the advantages of polyamide resins correspond to the disadvantages of ethylene copolymers while the advantages of ethylene copolymer correspond to the disadvantages of polyamide resins. Accordingly, many investigations have been made on blending the two types of resins into an alloy to compensate the mutual disadvantages and to improve the physical properties of the resins.
JP-B-55-44108, for example, describes blending of a small amount of an ethylene copolymer such as an ethylene-acrylic ester-maleic anhydride copolymer into a polyamide resin to improve the impact strength of the polyamide resin. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
JP-A-62-275157 proposes blending of a large amount of an ethylene-acrylic ester-maleic anhydride copolymer into a polyamide resin to produce a thermoplastic elastomer. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, in both cases, the products are compositions merely composed microscopically of separate phases, even though physical properties can be improved by combined use of a dispersant or other additives to increase the affinity between the two phases, because the products are produced mechanically by melt-blending a highly polar polyamide resin with an essentially less polar ethylene copolymer.
The present inventors have made extensive and detailed investigation from the viewpoint mentioned above and have found that copolymers are obtained which are improved drastically in bonding and dispersing level between the ethylene copolymer structure and the polyamide structure, and have excellent physical properties through graft copolymerization of a lactam compound onto an ethylene copolymer comprising specific components by a novel process.